As is known in the art, high frequency RF (radio frequency) signals having good frequency accuracy, spectral purity, and low phase noise are desired in many fields of electronics. To obtain the required frequency stability over temperature and time, these signals can be derived from oscillators, such as bulk acoustic wave (BAW), surface acoustic wave (SAW), microelectronic machined structure (MEMS) acoustic resonators, sapphire resonator oscillators (SRO), and opto-electronic-oscillators employing some form of optical delay.
The degradation of phase noise in vibrating environments is well known and is characterized by a parameter known as acceleration sensitivity. Acceleration sensitivity has tensor properties, but is generally characterized as a vector. Typical values range from 1×10−10/g to 1×10−9/g, where 1 g is an acceleration of 9.8 m/s2. Acceleration sensitivity is an issue in conventional oscillators.
As disclosed in R. J. Besson, et al., “Design of a Bulk Quartz Resonator Insensitive to Acceleration,” Proceedings of 33rd Annual Symposium on Frequency Control, 1979, when low frequency bulk-acoustic-wave resonators are mounted in a symmetrical structure, low acceleration sensitivity can be obtained. It was later discovered that perfect symmetry results in a resonator insensitive to acceleration, see, e.g., H. F. Tiersten, Y. S. Zhou, “On the normal acceleration sensitivity of contoured resonators stiffened by quartz cover plates supported by clips,” J. Appl. Phys. 72 (4) (1992), pp. 1244, 1254, and H. F. Tiersten, “On the Accurate Description of Piezoelectric Resonators Subject to Biasing Deformations,” Int. J. Engng Sci., Vol. 33, No. 15, pp. 2239-2259, 1995. Ten MHz bulk-acoustic-resonators having a high degree of symmetry and associated low acceleration sensitivity of 1×10−10/g are disclosed in U.S. Pat. No. 6,984,925, “Low acceleration sensitivity mounting structures for crystal resonators.” However, due to manufacturing and performance limitations, these methods are not applicable to resonators in the UHF or low microwave frequency ranges. Each of the above cited references is incorporated herein by reference.
Surface acoustic wave (SAW) resonators manufactured with semiconductor lithography processes are widely used in the UHF and low microwave frequency range. In J. A. Greer and T. E. Parker, “Improved Vibration Sensitivity of the All Quartz Package Acoustic Wave Resonator,” 42nd Annual Symposium on Frequency Control, 1988, it was demonstrated that lower acceleration sensitivity is achieved by fabricating SAW resonators on opposing sides of a quartz substrate. U.S. Pat. No. 5,345,201 to Greer at al., which is incorporated herein by reference, discloses such a symmetrical SAW device. However, this device was never fabricated or utilized outside of a laboratory environment due to difficulties in fabricating the resonators and using them in oscillator assemblies. Some of the difficulties include fabricating resonators on opposing surfaces of the same quartz wafer, obtaining matching electrical performance between the two resonators, and interfacing the opposing bonding pads on the resonator into the oscillator circuit. These difficulties prevented practical implementation of such devices.